At first blush “I Feel Pretty” has a promising premise. A sad duckling who’s far from ugly, only pleasantly plump and plagued by self-doubt, suddenly believes herself to be drop-dead gorgeous, and her delusion releases her joyous, lovable self. At second blush, after I saw the film, it’s a botched premise. At third blush, after I thought about what I’d seen, the premise never had a chance of working out well in the hands of filmmakers whose take on their subject is sitcom-deep.